the ring an eighth of a revolution. They will be shifted another eighth when the generator armature reaches the position shown in the last diagram, and will be successively displaced around the ring R as this armature revolves until a complete revolution has been made, when the parts are in their original position and ready to repeat the same cycle of operations.
The principle of the rotation of the magnetic poles has been applied by Mr. Tesla to a great variety of constructions. He has designed machines in which the field magnetism remains fixed and that of the armature is shifted, and others again in which there is a progressive shifting of the magnetic poles of both the field and armature in opposite directions. He has also found that the motor armature may consist of sets of closed coils, currents being developed in them by induction, and by making the induced portion of the generator stationary and the field revolving he has been able to produce apparatus free from all movable electrical contacts. In operating motors of this character Mr. Tesla usually employed a generator with multiple armature circuits as described above; but in the course of his experiments he discovered that the ordinary continuous or direct current machine could by slight alterations be made to furnish an alternating multiphase current as well as and in addition to the direct current. To accomplish this he found it was only necessary to add to the machine a pair of collector rings for each circuit of the multiphase current, and connect them with the proper armature coils. If, for instance, he desired to produce a two-phase current requiring two circuits from his generator to his motor, one circuit would include a set of coils in the armature of the generator that were passing through the position in which the maximum current was being produced, and the other a set of coils in which at the same time the minimum current was being generated. The phases of the current would then follow each other in the same order as in the previous machines with distinct circuits on the armature. With this form of machine a multiple phase alternating current, it will be seen, can be taken off from the collector rings, while a direct current can be taken from the commutator, and a part or the whole of this direct current be sent through the field coils to energize them and then put to any use for which such currents are suitable.
This machine was later developed into what has come to be known as a rotary transformer. Instead of being driven by power it is driven by one of the forms of current which it is capable of furnishing, the other current being taken off and utilized. For example, if a multiple-phase current is passed into the machine by the collector rings it will be driven as a motor and generate direct or continuous currents. If, on the